The invention relates to a method of forming a body of a formable working material in an impact machine, which comprises an upper impact unit with an upper ram, an upper punch, a lower impact unit with a lower ram, a lower punch, and a mould cavity for the working material between the punches when the punches are brought towards one another. The invention also relates to an impact machine for the carrying out of the method.
Impact machines for working by the employment of high kinetic energy are machines for working in the first place metal, such as cutting, punching, and plastic forming of powder-components, powder compaction, and similar operations, in which the speed of a ram, which may consist of a press piston, may be essentially higher than in conventional presses. Also polymeric and ceramic working materials can be conceived, as well as various composites of metals, polymers and ceramic materials. The working principle is based on the development of a very high kinetic energy of short duration instead of a high static press force of long duration. The dynamic forces of short duration which are generated at the ram impact and which in impact machines of prior art are conveyed around in the system via stand and foundation may be several thousand times larger than in conventional presses and imply that considerable amounts of energy are lost in stands and foundations instead of being used for effective work at an optimal way. In order to be able to absorb large force pulses, impact machines of prior art are equipped with very strong and heavy stands and foundations according to principles which are common in connection with conventional presses. Nevertheless the dynamic, shock type force pulses that are developed in impact machines are not damped in such heavy, conventional systems. The stress on all joints therefore become very large, as well as on sensitive components, e.g. electronic components for controlling those hydraulic valves which usually form parts of impact machines, which may cause a great risk of failure. Large, unwieldy stands also give rise to problems in connection with service, change of tool unit or of tool insets in the tool unit, changing the height of the impact unit above the tool unit, etc.
It is the purpose of the invention to address the above complex of problems. More particularly, the invention aims at achieving in the first place the following advantages:
to provide that the kinetic energy of rams can be used essentially for effective work in connection with the working of a working material instead of being lost in tools and auxiliary equipment, such as stand and foundation, which in turn can create improved possibilities to work and/or to form materials which previously have not been possible to be worked and/or formed to a desired degree,
to counteract shock waves from the impact via the machine stand, which in turn makes it possible to use a substantially lighter stand, and to eliminate heavy bases, which according to prior art have had the purpose of absorbing shock waves,
to reduce the total mass of the machine in comparison with known comparable impact machines, including reducing the sizes of the impact units, at the same time as lower striking velocities can be employed for the achievement of desired working, and
to make it possible to compact metal powders or other formable powder, such as ceramic powders or composite powders consisting mainly of metal, ceramic and/or polymeric powders, to a higher and more even density than what has been possible by means of prior art because of losses of energy in tools and auxiliary equipment.
The characteristic feature of the invention is that a body is formed of a formable working material in an impact machine comprising an upper impact unit with an upper ram, an upper punch, a lower impact unit with a lower ram, a lower punch, and a mould cavity for the working material, wherein downwards movable masses, which comprise at least the mass of the upper ram and the mass of the upper punch, and upwards movable masses, which comprise at least the mass of the lower ram and the mass of the lower punch, are caused to be moved towards one another and towards the working material in the mould cavity; that the masses which move downwards, including the upper punch, obtain a downwards directed velocity v1, and the masses which move upwards, including the lower punch, obtain an upwards directed velocity V2, wherein the movable parts have such masses and the velocities are so high that the momentum of the downwards movable masses and the upwards movable masses become essentially equal, i.e. such that the following condition applies:
m1xc3x97v1≅m2xc3x97v2
where m1 is the total mass of the masses moving downwards, and m2 is the total mass of the masses moving upwards plus the impact;
that the kinetic energies of the movable masses, i.e.                     m        1            ⁢              v        1        2              2    ,      and    ⁢          xe2x80x83        ⁢                            m          2                ⁢                  v          2          2                    2        ,
respectively, are essentially transferred to the working material in the mould cavity and are so great that the working material is plasticized and flows out to fill all parts of the mould cavity, when the punches are maximally brought together, for the formation of said body with desired shape.
According to a preferred method of carrying out the method, the body is formed of the formable working material in an impact machine which also comprises a central unit, said central unit including a die having a through hole which together with the upper punch and the lower punch form said mould cavity, said die being essentially stationary during the impact operation.
However, one can also conceive, according to an alternative embodiment, particularly for forming comparatively thin objects, that the machine has no central unit with a die, wherein the mould cavity is formed between the punches without any surrounding die, i.e. therein that one or both punches, preferably at least the lower one, is engraved, i.e. is embossed.
Prior to the impact, the upper punch and the lower punch preferably are pressed from above and from below, respectively, by a downwards directed force and by an upwards directed force, respectively, causing a static pressure from above and from below, respectively.
What is particularly characteristic according to an aspect of the invention, is that the upper and lower impact units can be raised and lowered in a controlled way for the setting of predetermined lengths of the upper ram and of the lower ram, respectively, prior to the forming operation in the case that the machine comprises an essentially stationary, central unit, wherein, according to this aspect of the invention, the upper and lower impact unit may be vertically adjustable in a controlled way relative to said central, essentially stationary unit.
Because of the short duration of the pressure pulse, when the material in the mould cavity is worked in a symmetrical way due to combined action by momentums from two directions, the risk that the high pressure will deform the punches and/or the die tool is reduced when the machine comprises a die, because of the inertia of the mass of the tool material. This allows for higher pressure in the mould cavity than what is possible according to prior art including forming by employing high kinetic energy, wherein the possibility to plasticize the working material in the mould cavity is improved. Because of the rapid material working, at least according to a conceived embodiment of the method and of the machine, and because of very short movements of the punches, also considerably less friction between the tool and working material will arise, and, in the case that the working material consists of powder, also less friction between the powder grains, which makes a considerably improved yield in the forming and the consolidation process possible, in other words that more energy can be brought to act on the working material. In an uncontrolled process there is a great risk that the tool will be damaged because of large friction forces.
It is thus a characteristic feature of the invention that a very high forming pressure is built up symmetrically from two directions and is allowed to act on the material that shall be shaped or consolidated with minimal losses due to friction, mass forces, and propagation of high frequent waves caused by impact initiated resonances in tool details and in surrounding machine, including the machine stand. Thus a very high forming pressure can be allowed to act in the tool chamber/mould cavity without breaking down the tool parts, because of an extremely short duration of the forming pulse and a great inertia of included tool parts.
A conceivable embodiment of the impact machine is characterized in that the upper impact unit comprises an upper impact body, that the lower impact unit comprises a lower impact body, that the upper punch and the lower punch are provided to be brought into contact with the working material in the mould cavity in the die prior to the forming operation, that the upper impact body and the lower impact body have, or are provided to be brought into direct or indirect mechanical contact with the upper punch and with the lower punch, respectively, at the latest in the functional positions of the impact bodies prior to the forming operation, that the length of the strokes of the rams correspond to the distance from the upper ram to the lower impact body when the lower impact body is in its functional position and the distance from the lower ram to the lower impact body when the lower impact body is in its functional position, respectively.
Said elevator devices for raising and for lowering the upper and lower impact units are suitably hydraulically working devices, wherein they can contribute to dampen shock waves which possibly may arise in connection with the forming operation, and may include at least two upper hydraulic lifting cylinders with vertical upper piston rods, which are included in an upper stand unit and carry an upper carrier for the upper impact unit; and at least two lower hydraulic lifting cylinders with vertical lower piston rods, which are included in a lower stand unit provided to carry a lower carrier for the lower impact unit. The piston rods form columns in the stand, which can be made very slender. Possibly, the number of such columns/piston rods with their sets of hydraulic cylinders may be increased to be more than two, however, suitably not more than four, in order to enhance the stability of the machine. Alternatively special guides may be provided to increase the stability, particularly the lateral stability. The hydraulic cylinders may be either stationary and fixedly united with the base or foundation, wherein the piston rods/columns are vertically adjustable relative to the base, or stationary. In the latter case, the piston rods suitably are designed as through piston rods in hydraulic cylinders which are mechanically connected with said carriers, which carry the impact units, as according to the principle described in the Swedish patent application No. 0001560-2 by the same applicant, the disclosure of said patent application being incorporated in the present patent application by reference.
When forming solid blanks, the blank should have a smaller extension in the radial direction than the tool cavity in order to make it possible to plasticize the blank before it contacts the side walls of the mould cavity. If an annular product shall be manufactured from a solid blank, the blank should have the shape of a ring, the outer diameter of which is smaller than the diameter of the tool cavity, and the inner diameter of which is larger than the diameter of the mandrel which in that case is provided in the centre of the tool, for the purpose of eliminating initial friction against the side wall of the mould cavity and against the mandrel.
In the foregoing description of the background art it is mentioned that impact machines employing high kinetic energy for working are machines in which the velocity of a ram, which can consist of a piston, can be substantially higher than in conventional presses. As a matter of fact this technique is often referred to as high velocity forming because high velocities of the impact members in impact machines generally have been considered to be a prerequisite for the achievement of intended results in terms of forming work. High velocities of the movable impact members, however, may imply a complication when working with those impact machines which are described in said Swedish patent applications, which machines work according to the counter striking pinciple, i.e. work with impact members which move towards one another during the striking/impact operation or with a lower, upwards movable anvil, which moves upwards, at the same time as upper impact members move downwards towards the movable anvil. The complication lies in the fact that the movements of the units which are movable towards one another must be synchronized and coordinated with great accuracy in terms of velocity (impulse) and position in order that the stroke shall be performed simultaneously with correct impulse of the masses which move towards one another, something which becomes increasingly difficult the higher the velocities of the moving parts are.
An aspect of the invention is based on the consideration that the velocities of the movable units in the impact machines, which move against one another during the impact operation, need not at all be as large as has been considered necessary because of what is taught by prior art. Nor should the kinetic energies need to be correspondingly high, i.e. lower velocity shall not necessarily need to be compensated by correspondingly larger movable masses. With the same masses, according to this aspect of the invention, the velocity thus can be reduced from about of 5 to 10 meters per sec. of said rams to the order of 1 meter per sec., or more generally 0.5-2 meters per sec.
The lower velocities thus improve the possibility to synchronize the movements of the movable units during the impact operation. Even though the velocities are radically reduced, the forming work nevertheless can be perfect, whether the working material is a powder or a solid body. Without binding the invention to any specific theory, it can be assumed that this is due to the good synchronization of the counter directed movements, which in turn has the result that the kinetic energy of the movable masses essentially can be used as effective forming work with small losses of energy to the machine foundation and stand.
Another favourable effect with the lower velocities of the movements that are movable towards one another, is that the ram travels can be shortened. This makes it possible to design the impact devices/the rams and the punches to form integrated units, as above mentioned. The punches in this case may be inserted into the upper and lower openings, respectively, of the die in the starting position for an impact operation, even if the punches are integrated with the impact devices/rams or corresponding, wherein the ram travels, i.e. the acceleration lengths, will be shorter than the axial length of the mould cavity of the die.
It shall thus be understood that the expressions high kinetic energy or very high kinetic energy are relative conceptions and shall be interpreted to mean adequate kinetic energy for the achievement of the effect in terms of forming work, which has been mentioned in the foregoing and which will be described more in detail in the following, detailed description of the invention.